when describing non-nullable method arguments.

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

Turns out that `[]:` means something in markdown, so you have to
escape the `:` with a backslash.

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Fixes #6382

Covers lib/ in package:flutter.

Since RelativeRect's whole purpose in life is to make work with Stack
easier, it's silly that you can't directly use it with Positioned.

Also, tests for RelativeRect.
And fixes for the bugs that found...

We use `@protected` in this way in StatelessWidget to avoid developers calling
this function directly.

See #4434.

I did a pass through some of the code cleaning minor things up.

Fixes #4761

Also, add more dartdoc to widgets.dart.

Turns out we were storing the same object in the base class and the
subclass. For pretty much every AnimatedWidget subclass.

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

Adds some names for common FractionalOffset values.

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Refactor Dismissable to not need a SizeObserver and to generally tighten
up the code a bit.

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.